Biocompatible low modulus medical implants

ABSTRACT

This invention relates generally to high strength, low modulus, metallic medical implants and, in particular, a titanium alloy useful in their fabrication which has a relatively low modulus of elasticity (e.g. closer to that of bone than other typically-used metal alloys) and which does not include any elements which have been shown or suggested as having short term or long term potential adverse effect from a standpoint of biocompatibility. The titanium alloy includes niobium and zirconium in specified amounts with tantalum an optional substitute for niobium.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 08/036,414filed Mar. 24, 1993, which is in turn a continuation-in-part of U.S.Ser. No. 07/986,280 filed Dec. 7, 1992, abandoned, which is acontinuation-in-part of Ser. No. 647,453; filed Jan. 28, 1991, U.S. Pat.No. 5,169,597 which is a continuation of U.S. Ser. No. 07/454,181 filedDec. 21, 1989, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to high strength, low modulus biocompatiblemedical implants made from titanium-niobium-zirconium alloys. Inparticular, implants of the invention have a modulus of elasticitycloser to that of bone than other typically-used metal alloys and do notinclude any elements which have been shown or suggested as having shortterm or long term potential adverse effect.

2. Background of the Invention

The most common materials used for load-bearing medical implants such aship or bone prostheses are metallic alloys, ceramics and compositesformed of biocompatible polymers and various reinforcing materials.

Polymers are typically used in implants such as intraocular lenses,facial bone remodeling and other non-load bearing applications. In orderto use plastic materials in load-bearing applications, they aretypically reinforced with a particulate or high modulus fibrousmaterial, such as carbon fiber, to produce composites of acceptablestrength capable of withstanding relatively great applied loads.Although composites are presently under consideration by many companies,their usefulness as an implant material lies in their relatively lowelastic modulus compared to metal and ceramic implants, and theiroptimum design characteristics are still being explored.

Ceramic prostheses provide excellent biocompatibility, corrosionresistance (inertness) and high compression strength for load-bearingapplications. However, ceramic prostheses are typically rigid (highelastic modulus), and unyielding under stress from loads applied duringnormal use, so that it cannot effectively transfer stresses tosurrounding bone. Thus, bone decalcification which results in localizedthinning (resorption) and weakening of the bone structure may occur.

Metals and metal alloys such as stainless steel, vitalium (cobalt alloy)and titanium have been used successfully. These materials have therequisite strength characteristics but typically have not been resilientor flexible enough to form an optimum implant material. Also, manyalloys contain elements such as aluminum, vanadium, cobalt, nickel,molybdenum, and chromium which recent studies have suggested might havesome long term adverse effects on human patients.

Many of the metal alloys typically used in prosthetic implants weredeveloped for other applications such as Ti-6A1-4V in alloy in theaircraft industry. These alloys were later thought to be suitable foruse as implant materials because they possess mechanical strength andappeared to have acceptable levels of biocompatibility. However, thesemetals typically have elastic moduli much higher than that of bone, forexample, 316 stainless steel has an elastic modulus of about 200 GPawhile that of cast heat-treated Co--Cr--Mo alloy is about 240 GPa. 0fthese, the alloy with the lowest elastic modulus is Ti-6A1-4V with anelastic modulus of about 120 GPa.

It has also been found that many of these metals will corrode to someextent in body fluids thereby releasing ions that might possibly beharmful over a prolonged period of time. It is now believed that thecorrosive effects of body fluids is due both to chemical andelectro-chemical processes, with corrosion products forming when certaincommonly-used metal alloys ionize from corrosion processes in the body.For example, aluminum metal ions have been associated with Alzheimer'sdisease and vanadium, cobalt, molybdenum, nickel and chromium aresuspected of being toxic or carcinogenic.

It has been suggested that metals could be coated with a biocompatibleplastic, ceramic or oxide to overcome the corrosion problem. However,coatings tend to wear off, especially on articulating bearing surfacesof total joints, and are susceptible to galling and separating from themetal substrate, exposing the metal to body fluids.

Generally, it is the industry practice to passivate the implant metalalloys. However, passivation produces only thin amorphous, poorlyattached protective oxide films which have not proved totally effectivein eliminating the formation of corrosion products in the body,particularly in situations where fretting occurs in the body.

Titanium alloys offer advantages over the stainless steels because oftheir lower susceptibility to corrosion in the body coupled with theirhigh strength and relatively low modulus of elasticity. Upon cooling,the currently used Ti-6A1-4V alloy transforms from a β-structure to anα- plus β-structure at about 1000° C. This transition can be shifted toa lower temperature by the addition of one or more suitable β-phasestabilizers such as molybdenum, zirconium, niobium, vanadium, tantalum,cobalt, chromium, iron, manganese and nickel.

Some efforts have been directed toward the development of alloys thateliminate harmful metals. For example, U.S. Pat. No. 4,040,129 toSteinemann et al. is directed to an alloy which includes titanium orzirconium as one component and, as a second component, any one or moreof: nickel, tantalum, chromium, molybdenum or aluminum, but does notrecognize or suggest any advantages from having a relatively low elasticmodulus, or advantages or disadvantages associated with high temperaturesintering treatments (at about 1250° C.), commonly employed to attachporous metal coatings into which bone can grow to stabilizenon-cemented, press-fit devices into the skeletal structure. Steinemannalso indicates that the ultimate tensile strength should be greater thanabout 979 MPa (142 ksi) with a minimum tensile elongation of 10%.

Although Steinemann provides that copper, cobalt, nickel, vanadium andtin should be excluded, apart from their presence as unavoidableimpurities, the patent indicates that it is permissible to have any orall of chromium, molybdenum and aluminum, which are all believed to havepotential long-term adverse effects, present in the alloy as long astheir combined weight does not exceed 20% of the total weight of thealloy.

U.S. Pat. No. 4,857,269 to Wang et al. is not a statutory bar and itscitation is not an admission that its teachings are applicable priorart. This patent relates to a titanium alloy for a prosthetic implantsaid to have high strength and a low modulus. The titanium alloycontains up to 24 wt. % of at least one isomorphous beta stabilizer fromthe group molybdenum, tantalum, zirconium and niobium; up to 3 wt. % ofat least one eutectoid beta stabilizer from the group iron, manganese,chromium, cobalt or nickel; and optionally up to 3 wt. % of a metallicα-stabilizer from the group aluminum and lanthanum. Incidentalimpurities up to 0.05% carbon, 0.30% oxygen, 0.02% nitrogen, and up to0.02% of the eutectoid former hydrogen are also included. Although thereis some discussion of having an elastic modulus (e.g., Young's modulus)around 85 GPa, the only examples of a low modulus (66.9-77.9 GPa) allcontain 11.5 wt. % Mo which is a potentially toxic element andundesirable for optimizing biocompatibility.

Other currently used metal alloys have similar drawbacks. For example,the commonly used Ti-6A1-4V alloy, with appropriate heat treatment,offers some degree of biocompatibility but has an elastic modulus ofabout 120 GPa. Although this elastic modulus is lower than other alloysand accordingly offers better load transfer to the surrounding bone,this modulus is still significantly greater than desired. Moreover, thealloy contains aluminum and also vanadium, which is now suspected to bea toxic or carcinogenic material when present in sufficient quantity.

Commercially available PROTOSUL 100 (Sulzer Bros. Ltd.) is a Ti-6Al-7Nballoy which intentionally avoids the potentially adverse effects ofvanadium toxicity by substituting niobium. However, the alloy stillcontains aluminum and has an elastic modulus of about 110 GPa (15.9×10psi) in heat-treated condition, and with a tensile strength of about1060 MPa.

With orthopedic prostheses being implanted in younger people andremaining in the human body for longer periods of time, there is a needfor an implant material with requisite strength and flexibilityrequirements, which does not contain elements which are suspected ashaving long-term harmful effects on the human body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a hip joint stem with a porous coating.

FIG. 2 is a bar graph comparing the mechanical properties of aTi--Nb--Zr alloy useful in the invention with other materials and bone.

FIG. 3 is a bar graph comparing the mechanical properties of aTi--Nb--Zr alloy useful in the invention with other materials and bone.

FIGS. 4A and 4B shows the effect of various techniques of age hardeningon two different Ti--Nb--Zr alloys useful in the invention.

SUMMARY OF THE INVENTION

The invention is of implants made from a material which possesses thecharacteristics of relatively high strength, exceptionally low modulusof elasticity, and is free from any potentially toxic elements. Theuseful alloy contains about 74 wt. % titanium, and about 13 wt. % eachof zirconium and niobium. Other elements are not deliberately added, butmay be present in trace amounts to the extent that they were present asunavoidable impurities in the metals used to produce the alloy. Othernon-toxic filler materials such as tantalum, which could be used tostabilize the β-phase, but not affect the low modulus, i.e. maintain itless than about 85 GPa, could also be added. The exclusion of elementsbeside titanium, zirconium and niobium or tantalum results in an alloywhich does not contain known toxins or carcinogens or elements that areknown or suspected of inducing diseases or adverse tissue response inthe long term.

The titanium alloys useful in the invention are rapidly cooled fromabove the β-transus and aged to provide adequate strength. Further, thealloys have a low modulus of elasticity, even after high-temperaturesintering to attach porous-coatings, of about 62-75 GPa. This comparesfavorably with the elastic modulus of fiber reinforced polymercomposites, which are typically in the range 60-70 GPa for strengthadequate for long-term in-vivo loading, and is a significant improvementover Ti-6A1-4V which has a modulus of elasticity of about 120 GPa.

A comparison of the mechanical properties of the invention Ti-13Zr-13Nballoy with other implant materials is shown in FIG. 2 where the elasticmodulus of alumina is represented by the bar marked 21, zirconia by 22,cobalt-chrome-molybdenum by 23, 316 SS by 24, Ti-6A1-4V by 25,Ti-13Zr-13Nb by 26, a composite of polyetheretherketone and carbon fiberby 27 and cortical bone by 28. Further, the mechanical properties of theinvention alloy implants are compared with other alloys in FIG. 3 whereTi-13Zr-13Nb is represented by 29, 316 SS (30% CW) by 30, castcobalt-chrome-molybdenum by 31, 316 SS by 32, Ti-13Zr-13Nb by 33, acomposite of polyetheretherketone and carbon by 34, a carbon polysulfonecomposite by 35, and cortical bone by 36.

In certain applications it may still be desirable to coat the surfacewith wear-resistant coatings such as amorphous diamond-like carboncoatings, zirconium dioxide coatings, titanium nitrides, carbides, orthe like for protection against potential micro-fretting, such as mightoccur on the bearing surfaces of implant prostheses.

A porous coating, such as a bead or wire mesh coating, as exemplifiedschematically in FIG. 1 as 10 on hip stem prosthesis 20, may be appliedto implants of many types for a variety of applications fabricated fromthe inventive alloy. Such coatings are often useful to provideinterstitial spaces for tissue ingrowth into the implant, which tends tostabilize the implant in the skeletal structure. Further, even thoughthe application of such porous coatings usually requires sintering atrelatively high temperatures, the properties of the alloy that mightaffect its usefulness as an implant material are not adversely affected.

While implants according to the invention fabricated fromtitanium-niobium-zirconium alloy possess a relatively high strength, theusefulness of these implants is not limited to load-bearingapplications. Because of its corrosion resistance and non-toxicity andrelatively low modulus of elasticity, the alloy can be used to fabricatemany types of implants including, but not limited to, hip joints, kneejoints, cheek bones, tooth implants, skull plates, fracture plates,intramedullary rods, staples, bone screws, and other implants.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The implants of the invention may be produced by combining, ascommercially pure components, titanium, zirconium and niobium in theappropriate proportions. The methods for titanium alloy production, suchas casting, powder metallurgy, etc., are well known to those of ordinaryskill in the art of metallurgy and the production of the alloy requiresno special skills or precautions beyond the materials, proportions andtechniques described below.

The alloys useful in the invention contain titanium as the majorcomponent comprising about 74 wt. % of the alloy in combination withabout 13 wt. % of zirconium and 13 wt. % of niobium. While tantalum maybe substituted for niobium to stabilize the β-phase titanium, niobium isthe preferred component due to its effect of lowering the elasticmodulus of the alloy when present in certain specific proportions. Otherelements are not deliberately added to the alloy but may be present insuch quantities that occur as impurities in the commercially puretitanium, zirconium, niobium or tantalum used to prepare the alloy andsuch contaminants as may arise from the melting (alloying) process.Non-toxic filler materials, such as tantalum, could also be added toreduce the β-transus (stabilize β) and improve strength as long as therelatively low modulus of elasticity (less than about 85 GPa) of thebase alloy is not significantly affected.

While the as-cast or powder metallurgically prepared alloy can be usedas an implant material, it can optionally be mechanically hot rolled at825°-875° C. After cooling, it can then be reheated to about 875° C. forabout 20 minutes and then quenched with water. This reheating step maybe eliminated if the alloy is quenched rapidly from the hot workingtemperature. These hot rolling, cooling, reheating and quenching stepsdevelop the cast alloy into a wrought material having a finer grain thanthe as-cast or powder metallurgically prepared alloy and renders it moresuitable for use as an implant material.

The alloy, in this hot rolled, reheated and quenched form, has anelastic modulus of about 60 GPa, a tensile strength of about 700 GPa andan elongation of about 25%. While such an alloy might be suitable foruse in a variety of implant applications, it is desirable that alloysused in more severe load-bearing implant applications have a greaterstrength as well as a low elastic modulus (less than about 85 GPa).

In the specification and claims, the term "high strength" refers to analloy having a tensile strength above at least about 620 MPa.

The term "low modulus" as used in the specification and claims refers toa Young's modulus below about 85 GPa.

Although the hot rolled, reheated and quenched alloy is a suitableimplant material, its properties can be improved by forging or coldworking or an aging heat treatment or a combination of these. Agingtreatment can increase the strength and hardness of the material, andreduce its elongation while maintaining a relatively low modulus ofelasticity. The treatment can be varied to obtain the desiredproperties.

In titanium alloys, the niobium (or tantalum, if this element is added)acts to stabilize the β-phase since it is a β-isomorphous phasestabilizer. This results in a lower β-phase transus temperature and uponrapid cooling from about the β-transus temperature, the presence of agreater proportion of the β-phase titanium in the alloy microstructure.This enhances the ability of the alloy to harden on subsequent aging.

Niobium, in particular, when present in preferred quantities of fromabout 6 to about 10 atomic percent (most preferably about 8 atomicpercent) or in an alternative preferred range of from about 22 to 32atomic percent, produces a low modulus composition when alloyed withtitanium. Deviation from these ranges of niobium concentration tends toincrease the elastic modulus. In weight percent terms, these preferredcompositional ranges of niobium in the titanium-zirconium alloytranslate to about 10 to about 20 wt. % and about 35 to about 50 wt. %.

Titanium alloys containing about 13 wt. % niobium correspond to thosehaving about 8 atomic percent niobium. Thus, the Ti-13Nb-13Zr alloy isbelieved to identify an optimal low modulus, titanium alloy composition.

As previously mentioned, tantalum may be substituted for niobium tostabilize the β-phase, but niobium is preferred due to its effect inreducing the elastic modulus. Substitution with zirconium can improvestrength.

Whereas the niobium proportion is critical to obtain the desired lowmodulus property, the zirconium proportion is not as critical. It isdesirable to maintain the proportion of zirconium at less than about 20wt. % but higher proportions are also useful.

Zirconium, it is believed, is capable of stabilizing both α- and β-phasetitanium alloy, but acts by being in solution in the alloy as aβ-stabilizer by slowing the transformation process in the inventivealloy. It is further believed that the larger ionic radius of zirconium(35% larger than that of titanium) helps to disrupt ionic bonding forcesin the alloy resulting in some reduction in the modulus of elasticity.

In order to effect the transition to the β-phase, the alloy may betreated by heating to about 875° C. for about 20 minutes. Lowertemperatures above the β-transus may also be used. The β-phase may alsobe induced by heating to higher temperatures for shorter periods oftime. The critical factor is heating to at least about the β-transitiontemperature, about 728° C. for Ti-13Zr-13Nb, for a period of timesufficient to obtain a substantial conversion of the titanium alloy tothe β-phase prior to cooling to room temperature. Conversion of thealloy to the β-phase and cooling may be effected before during, or aftershaping for implantation and sintering of a porous metal coating,whichever is most convenient.

The effect of hardness and aging conditions for Ti-13Zr-13Nb andTi-18Zr-6Nb alloys cooled at two different rates from above the betatransus are shown in FIGS. 4A and 4B. In FIG. 4A, a water quench is usedwhereas in FIG. 4B, air cooling is employed.

Based upon the foregoing, it is apparent that the titanium proportion ofcertain embodiments of the alloy could be less than 50 wt. %.Nevertheless, these alloys are, for purposes of the specification andclaims, referred to as "titanium alloys." For example, a titanium alloymay contain 20 wt. % zirconium and 45 wt. % niobium with only 35 wt. %titanium.

The machining, casting or forging of the alloy into the desired implantshape may be carried out by any of conventional methods used fortitanium alloys. Further, implants could be pressed from the powderedalloy under conditions of heat and pressure in preforms in the shape ofthe desired implant. Conventional sintering and hot isostatic pressuretreatments can be applied.

While the alloy provides a non-toxic prosthesis, it may yet be desirablefor other reasons such as micro-fretting against bone or polyethylenebearing surfaces to coat the prosthesis. In this event, the surface ofthe prosthesis may be coated with an amorphous diamond-like carboncoating or ceramic-like coating such as titanium nitride or titaniumcarbide using chemical or plasma vapor deposition techniques to providea hard, impervious, smooth surface coating. These coatings areespecially useful if the prosthesis is subjected to conditions of wear,such as, for instance, in the case of bearing surfaces of knee or hipprostheses.

Methods for providing hard, low-friction, impervious, biocompatibleamorphous diamond-like carbon coatings are known in the art and aredisclosed in, for example, EPO patent application 302 717 A1 to Ion Techand Chemical Abstract 43655P, Vol. 101 describing Japan Kokai 59/851 toSumitomo Electric, all of which are incorporated by reference herein asthough fully set forth.

Implants according to the invention fabricated from the Ti--Nb--Zr alloymay be supplied with a porous bead or wire coating of titanium alloy ofthe same or different composition including pure titanium to allowstabilization of the implant in the skeletal structure of the patientafter implantation by bone ingrowth into the porous structure. Suchporous structures are normally attached to the implant surface bysintering. This involves heating the implant to above about 1250° C. Themechanical properties of titanium alloys can change significantly due tosubstantial grain growth and other metallurgical factors arising fromthe sintering process. Thus, after sintering to attach the porouscoating, it is preferred that the Ti-13Zr-13Nb implant be reheated toabout 875° C. (or above the β-transus) for 20-40 minutes then quenchedbefore being aged at about 500° C. for about 6 hours to restoremechanical properties. If quenched adequately from the sinteringtemperature, it may be possible to go directly to the aging process.

The following examples are intended to illustrate the invention asdescribed above and claimed hereafter and are not intended to limit thescope of the invention in any way. The aging temperature used in theexamples is determined to be acceptable, but not necessarily optimal,based on the hardness versus aging response show in FIGS. 4A and 4B.

EXAMPLE 1

An alloy including, by weight, 74% titanium, 13% niobium and 13%zirconium, was hot rolled at a temperature in the range 825°-875° C. to14 mm thick plate. The plate was cooled to room temperature thenreheated to 875° C. where it was maintained for 20 minutes and thenwater quenched to room temperature. The β-transus for this alloy wasabout 728° C. as compared to about 1000° C. for Ti-6A1-V. The mechanicalproperties of the heat-treated, quenched Ti--Zr--Nb alloy, which has anacicular transformed β-structure, are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Mechanical Properties of Ti--13Zr--13Nb                                       As Water Quenched from Hot Rolling Temperature                                ______________________________________                                        Tensile Strength  710 MPa                                                     Yield Strength    476 MPa                                                     Elongation        26%                                                         Reduction in Area 70%                                                         Young's Modulus    62 GPa                                                     Rockwell C Hardness                                                                             18-19                                                       ______________________________________                                    

EXAMPLE 2

The heat-treated, quenched Ti--Zr--Nb alloy of Example 1 was aged byheating at 500° C. for 6 hours. The mechanical properties of this agedalloy are shown in Table II.

                  TABLE II                                                        ______________________________________                                        Mechanical Properties of Quenched                                             :Ti--13Zr--13Nb Aged 500° C. for Six Hours                             ______________________________________                                        Tensile Strength  917 MPa                                                     Yield Strength    796 MPa                                                     Elongation        13%                                                         Reduction in Area 42%                                                         Young's Modulus   76.6 GPa                                                    Rockwell C Hardness                                                                             About 29                                                    ______________________________________                                    

EXAMPLE 3

Samples of the alloy of Example 1 were sintered at about 1250° C. toattach a porous titanium bead coating of the type shown in FIG. 1. Thebead-coated alloy samples were then reheated to 875° C. and maintainedat this temperature for 40 minutes before being water-quenched. A groupof six samples were aged at 500° C. for 6 hours and the mechanicalproperties of aged and non-aged samples (three each) were tested and areshown in Table III.

                  TABLE III                                                       ______________________________________                                        Mechanical Properties of Ti--13Zr--13Nb Alloy                                 Following Bead Sintering, Reheating to                                        :875° C., and Water Quenched                                                    As-quenched (Avg.)                                                                        Aged (500° C. Six Hours                           ______________________________________                                        Tensile Strength                                                                         664 MPa       900 MPa                                              Yield Strength                                                                           465 MPa       795 MPa                                              Elongation 20%           4%                                                   Reduction Area                                                                           46%           9%                                                   Young's Modulus                                                                          61.8 GPa      74.7 GPa                                             ______________________________________                                    

Note that the sintering treatment can significantly alter the mechanicalproperties, particularly ductility. Thus, an alloy acceptable for aparticular application in unsintered form may not necessarily beeffective in that application following a high-temperature sinteringtreatment routinely used to attach a porous titanium coating. Tominimize these effects, lower temperature diffusion bonding methods canbe used in which a sintering temperature near the β-transus may beeffective. Alternatively, pre-sintered porous metal may be effective.Alternatively, pre-sintered porous metal pads can be tack-welded to theimplant.

EXAMPLE 4

A comparison of the elastic modulus, tensile strength and yield strengthof the Ti-13Zr-13Nb invention alloy with those of known alloys,composites and cortical bone, are summarized in FIGS. 2 and 3. Al₂₀ 3and ZrO₂ refer to ceramics while C/PEEK refers to a carbon reinforcedpolyetheretherketone composite and C/PS refers to a carbon reinforcedpolysulfone composite. All the mechanical property data of FIGS. 2 and 3were obtained from literature sources except for the data pertaining tothe invention alloy which were measured using standard ASTM tensiletesting techniques. It is significant that the Ti-13Zr-13Nb inventionalloy has an elastic modulus similar to carbon fiber reinforcedcomposites and closer to that of bone than the other metals (FIG. 2)while at the same time possession a strength comparable to or betterthan other metals (FIG. 3).

EXAMPLE 5

A sample of Ti-18Zr-6Nb was sintered to attach a porous metal costing.Thereafter, the sintered alloy was reheated to 875° C., i.e. above theβ-transus, and water quenched. The properties of the as-quenched alloyare shown in Table IV. The same was then aged at 450° C. for 3 hours andtested. These results are also shown in Table IV.

As compared to the Ti-13Zr-13Nb alloy of Example 3, this alloy's modulusof elasticity is not as low but is still lower than that of Ti-6A1-4V.Further, the Ti-18Zr-6Nb alloy has a relatively low β-transus, about760° C. compared to that of Ti-6A1-4V which is about 1000° C.

                  TABLE IV                                                        ______________________________________                                        Mechanical Properties of Ti--18Zr--6Nb Following A                            High Temperature Sintering Treatment, Reheating to                            875° C., and Water Quenching and Aging                                            As Quenched                                                                              Aged 450° C., 3 Hrs.                             ______________________________________                                        Tensile Strength                                                                            807 MPa      876 MPa                                            Yield Strength                                                                              659 MPa      733 Mpa                                            Elongation    8%            8%                                                Reduction in Area                                                                          26%            28%                                               Elastic Modulus                                                                            85.2 GPa     86.8%                                               ______________________________________                                    

Note that because of the less than optimum niobium content, the elasticmodulus is not as low as the previous example. Thus, proper selection ofniobium content is important for optimizing the low elastic modulus.However, the presence of zirconium helps to keep the elastic modulus atan acceptably low level (less than about 85 GPa).

EXAMPLE 6

The effect of aging conditions on Ti-13Zr-13Nb and Ti-18Zr-6Nb wasinvestigated. Separate samples of each alloy were air-cooled orwater-quenched from above the β-transus, aged at 500, 450, 400 and 350°C. for up to 6 hours then air cooled. The results are recorded in FIGS.4A and 4B.

The invention has been described with reference to its preferredembodiments. From this description, a person or ordinary skill in theart may appreciate changes that could be made in the invention which donot depart from the scope and spirit of the invention as described aboveand claimed hereafter.

What is claimed is:
 1. A biocompatible metallic implant, the implant atleast partially fabricated from an alloy consisting essentially of thefollowing alloying components:(a) titanium; (b) from about 10 to about20 wt. % as the sum of metals selected from the group consisting ofniobium and tantalum; and (c) an amount of zirconium sufficient to actas a beta stabilizer by slowing the transformation of beta;wherein toxicelements are excluded except for those trace amounts of impurities andinterstitials present in the alloying components or picked up duringfabrication of the implant.
 2. The implant of claim 1, wherein the alloyhas an elastic modulus of from about 60 to about 90 GPa.
 3. The implantof claim 1, wherein the alloy includes about 74 wt. % titanium, about 13wt. % niobium and about 13 wt. % zirconium.
 4. The implant of claim 1wherein the alloying components are titanium, about 10 to about 20 wt. %niobium, and less than about 20 wt. % zirconium.
 5. The implant of claim1, wherein the alloy has a modulus of elasticity of less than about 85GPa.
 6. The implant of claim 1 further comprising a wear resistantcoating on outer surfaces.
 7. A low modulus, biocompatible, metallicimplant for implantation in a human body, the implant at least partiallyfabricated from an alloy consisting of:(a) titanium; (b) from about 10to about 20 wt. % as the sum of metals selected from the groupconsisting of niobium and tantalum; and (c) an amount of zirconiumsufficient to act as a beta stabilizer by slowing the transformation ofbeta.
 8. The implant of claim 7 wherein the alloy consists of 74 wt. %titanium, 13 wt. % niobium, and 13 wt. % zirconium.
 9. The implant ofclaim 7 further comprising a wear resistant coating on surfaces.
 10. Theimplant of claim 7 wherein the zirconium amount is less than about 20wt. %.
 11. A biocompatible, metallic implant for implantation in a humanbody, the implant at least partially fabricated from an alloy consistingessentially of the following alloying components:(a) titanium; (b) fromabout 35 to about 50 wt. % of the sum of metals selected from the groupconsisting of niobium and tantalum; and (c) an amount of zirconiumsufficient to act as a beta stabilizer by slowing the transformation ofbeta;wherein toxic elements are excluded except for those trace amountsof impurities and interstitials present in the alloying components orpicked up during fabrication of the implant.
 12. The implant of claim 11wherein the amount of zirconium is less than about 20 wt. %.
 13. Theimplant of claim 11 wherein the modulus of elasticity of the alloy isless than about 85 GPa.
 14. The implant of claim 11 further comprising awear resistant coating on outer surfaces.